Low temperature co-fired substrate composition

ABSTRACT

It is demanded that a LTCC substrate composition capable of maintaining low relative permittivity k and high Q value without having a reactivity with a silver which is an electrode material and causing migration of the silver during a co-firing operation at a low temperature. Provided with a low temperature co-fired substrate composition containing 83 to 91 wt. % of CaO-B 2 O 3 -SiO 2  based glass powder, 7.5 to 14 wt. % of two or more kinds of nanometer-sized SiO 2  powders having different ranges of particle diameter and 1.5 to 3 wt. % of β-wollastonite powder as a crystallization agent wherein the glass powder contains 40.0 to 45.0 wt. % of CaO, 9.0 to 20.0 wt. % of B 2 O 3  and 40.0 to 46.0 wt. % of SiO 2 .

TECHNICAL FIELD

The present invention relates to a low temperature co-fired (simultaneously fired) ceramic (LTCC) substrate composition mainly used for a small size integrated module for wireless communication.

BACKGROUND ART

Tablet terminals including a mobile phone and a smartphone have spread rapidly. A lot of circuit chips are integrated into the above described terminals. In particular, low temperature co-fired (simultaneously fired) ceramic (LTCC: Low Temperature Co-fired Ceramics) substrates are widely used for front-end modules. The front-end means an element directly communicates with the outside for displaying the information for the user, receiving an instruction and inputting/outputting the information with the other systems, for example. The front-end module means a very small size integrated module integrated with various functional components used for wireless front-end circuit such as LTE which is one of communication standards of the mobile phone, Wi-Fi and GPS. The LTCC substrate is a kind of ceramics used for electronic materials. This substrate is called as the LTCC because it is formed by simultaneously burning a ceramic carrier structure, a conductive resistor and a dielectric material in a furnace of less than 1000° C. The LTCC substrate is used as an electronic circuit substrate by integrating a capacitor, a resistor and the like on one layer and combining a plurality of layers.

In recent years, devices such as a high speed wireless device, a vehicle collision avoidance radar, a dangerous substance detection device and the like have been developed actively in the range of 30 to 300 GHz called a millimeter wave. The demand for the LTCC substrate used for the above described high frequency communication has been increased. It is known that the millimeter wave has extremely high straight advancing properties and large information transmission amount. As for the material characteristic of the LTCC substrate capable of utilizing above described properties, it is required to minimize the signal attenuation and to propagate the signal at high speed. From the above described background, the demand for the characteristics required for the substrate material used for the LTCC substrate have been increased.

Here, the signal attenuation in the high frequency region is a sum of conductor loss and dielectric loss. In the region exceeding 1 GHz, the signal attenuation caused by the dielectric loss of the LTCC substrate is dominant. In general, the dielectric loss of the dielectric body is represented by dielectric tangent (dielectric loss tangent) tan δ. However, the dielectric loss is also represented by a Q value (=1/tan δ) instead of the dielectric tangent in many cases. In addition, it is known that the relation V˜(k^(−1/2)) is satisfied when a propagation speed of the signal is V and a dielectric constant is k. Accordingly, in order to maximally utilize the characteristics of the millimeter wave communication (transmit signals at high speed), the material having high Q value (High Q) and low dielectric constant (Low k) is required.

As the dielectric loss mechanism of the glass material (i.e., LTCC substrate) in the high frequency region such as the millimeter wave, a large part is occupied by deformation loss caused when mesh structure forming the glass is deformed by an electric field to cause dipolar orientation and vibration loss caused when synchronized with the applied frequency. The above described losses generally increase in proportion to the frequency. It is known that the deformation and the vibration loss are smaller in a crystal material compared to the glass which is an amorphous material since the crystal material has a stronger structure capable of reducing the deformation and the vibration. In order to reduce the above described two losses as much as possible, the method of adding a high purity filler to the glass composition such as borosilicate glass having High Q and the method of synthesizing a crystal material having High Q by a crystallized glass method can be listed.

The former method is disclosed in Patent Document 1, for example. Patent Document 1 discloses a powder of alkali borosilicate glass having a low relative permittivity and a low dielectric loss containing Li₂O, Na₂O and K₂O in a predetermined molar ratio, discloses a glass-ceramic dielectric material formed of the above described powder and 20 to 50 wt. % of ceramic filler powder and discloses a sintered body formed by sintering the glass ceramic dielectric material. In Patent Document 1, an amorphous glass not depositing crystal is preferably used for the alkali borosilicate glass since the amorphous glass has good softening fluidity when it is sintered and a dense sintered body can be obtained easily.

The latter method seems to be disclosed in Patent Document 2. Patent Document 2 describes to provide a noncrystal glass/glass ceramic containing a crystallized glass formed from CaO-B₂O₃-SiO₂ base having the composition of approximately 18 to 32 wt. % of B₂O₃ approximately 42 to 47 wt. % of CaO and approximately 28 to 40 wt. % of SiO₂ and a noncrystal glass having the composition of at least approximately 60 wt. % of SiO₂, approximately 10 to 35 wt. % of B₂O₃ and up to approximately 6 wt. % of alkali metal oxide.

Patent Document 3 discloses a sintered dielectric material containing a solid part containing a silica powder and a multicomponent amorphous glass before sintering and discloses the sintered dielectric material further containing a crystalline compound selected from the group consisting of a wollastonite, a calcium borate and the like.

The purpose of the inventions of the above described cited references is to provide a co-fired substrate composition capable of being fired at a low temperature of 1100° C., 1000° C. or less and having a low relative permittivity k and a high Q value (low dielectric tangent tan δ) at a high frequency region exceeding 1 GHz.

CITATION LIST Patent Literature

Patent Document 1: Japanese Unexamined Patent Application Publication No. 2004-269269

Patent Document 2: Japanese Unexamined Patent Application Publication No. H5-238813

Patent Document 3: Japanese Unexamined Patent Application Publication No. 2019-108263

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

As described in the background art, the LTCC substrate composition having a low relative permittivity k and a high Q value has been pursued. However, as another important characteristic of the LTCC substrate, it is required not to have reactivity with the silver (Ag) which is an electrode material during a co-firing operation at a low temperature, not to cause the migration of silver and not to generate warpage in the LTCC substrate.

Patent Document 3 describes that high Q value and low relative permittivity k can be shown when only the glass component is fired without adding the silica powder but also describes that it is considered that the problems may occur when co-fired with a silver conductor for the purpose of the LTCC since the silver moves largely. In addition, Patent Document 3 describes that the addition of the silica powder to the glass component may cause the ability of adjusting the contraction of the dielectric material and the ability of control the movement of the silver in the dielectric material during the firing. However, the configuration and the effect are not revealed in Patent Document 3.

Means for Solving the Problem

In order to solve the above described conventional problems, the present invention provides a low temperature co-fired substrate composition before a co-firing process, comprising:

-   -   (A) 83 to 91 wt. % of glass powder containing CaO-B₂O₃-SiO₂ as a         basic composition;     -   (B) 7.5 to 14 wt. % of SiO₂ powder; and     -   (C) 1.5 to 3 wt. % of β-wollastonite (CaSiO₃) powder, wherein     -   a particle diameter D₅₀ of the glass powder (A) is 2.0 to 3.0         μm,     -   the glass powder (A) contains 40.0 to 45.0 wt. % of CaO, 9.0 to         20.0 wt. % of B₂O₃ and 40.0 to 46.0 wt. % of SiO₂ as a         composition,     -   the SiO₂ powder (B) is a filler selected from the group         consisting of (1), (2) and (3):         -   (1) 10 to 30 wt. % of the SiO₂ powder having the particle             diameter D₅₀ of 10 to 100 nm and 70 to 90 wt. % of the SiO₂             powder having the particle diameter D₅₀ of 400 to 3000 nm;         -   (2) 60 wt. % or more of the SiO₂ powder having the particle             diameter D₅₀ of 100 to 400 nm and less than 40 wt. % of the             SiO₂ powder having the particle diameter D₅₀ of 400 to 3000             nm; and         -   (3) 4 to 20 wt. % of the SiO₂ powder having the particle             diameter D₅₀ of 10 to 100 nm, 60 to 95 wt. % of the SiO₂             powder having the particle diameter D₅₀ of 100 to 400 nm and             0 to 36 wt. % of the SiO₂ powder having the particle             diameter D₅₀ of 400 to 3000 nm,     -   the β-wollastonite (CaSiO₃) powder (C) is another filler having         the particle diameter D₅₀ of 2.0 to 3.0 μm,     -   when a silver paste, which functions as an electrode, is applied         on a surface of a green sheet comprised of the low temperature         co-fired substrate before the co-firing process and an organic         binder and the co-firing process is performed, the low         temperature co-fired substrate after the co-firing process has         the following properties:

a relative permittivity k is 6.0 or less at 2.5 GHz;

-   -   a Q value which is a reciprocal of dielectric tangent is 500 or         more at 2.5 GHz; and     -   a ratio R₄₂₀/R₈₀₀ of a reflectance R₄₂₀ in a wavelength of 420         nm with respect to a reflectance R₈₀₀ in the wavelength of 800         nm is 85% or more.

Although the low temperature co-fired substrate composition before the co-firing process contains (A) 83 to 91 wt. % of glass powder containing CaO-B₂O₃-SiO₂ as a basic composition; (B) 7.5 to 14 wt. % of SiO₂ powder; and (C) 1.5 to 3 wt. % of β-wollastonite (CaSiO₃) powder as necessary compositions, it can also contain less than 3 wt. % of an oxide such as ZrO₂ having high chemical stability as other compositions.

In the present invention, the concentration of the glass powder is necessarily 83 to 91 wt. %. When the concentration exceeds 91 wt. %, the relative permittivity k is too large exceeding 6.0. On the other hand, when the concentration is less than 83 wt. %, the adding amount of the SiO₂ powder is too large and the sinterability of the co-firing is deteriorated. As a result, the dielectric tangent tan δ is too large and the Q value is lower than 500.

Although the glass powder (A) contains 40.0 to 45.0 wt. % of CaO, 9.0 to 20.0 wt. % of B₂O₃ and 40.0 to 46.0 wt. % of SiO₂ as the composition, the glass powder (A) can also contain less than 2 wt. % of an oxide such as Al₂O₃, ZrO₂ and MgO having high chemical stability. The particle diameter D₅₀ of the glass powder is 2.0 to 3.0 μm. In accordance with the downsizing of the ceramic chip component, the sheet thickness (film thickness) of the LTCC substrate becomes thinner to 50 μm or less. The film thickness of the conductive paste to be co-fired is mainly 10 μm or less. When the particle diameter D₅₀ of the glass powder is larger than 2.0 to 3.0 μm, roughness of the LTCC substrate after the co-firing process is increased and the electrode to be co-fired is badly affected. On the other hand, when the particle diameter D₅₀ of the glass powder is less than 2.0 to 3.0 μm, the specific surface area is increased. Thus, in order to uniformly disperse the glass powder, the amount of the organic binder should be increased. This causes the problem that malfunction increases in the layered structure due to degreasing defects.

The basis for choosing the above described composition of the glass powder will be explained using Table 1.

The compositions shown as (a), (b), (c) and (d) in the field of the glass composition code are the glass compositions within the range of the claim of the present invention. The above described compositions have low relative permittivity k, low dielectric tangent (high Q) and not so high crystallization temperature Tc. Since there is a certain temperature difference between the crystallization temperature Tc and the glass transition temperature Tg, workability is good. On the other hand, the glass code number OG302 and OG303 do not satisfy the target values of the present invention since the concentration of SiO₂ is lower than 40.0 wt. %, the concentration of B₂O₃ is higher than 20.0 wt. % and the dielectric tangent (tan δ) is not 0.002 or less (Q valued is 500 or more). In the glass code number OG305, the concentration of CaO is slightly lower than 40 wt. % and problems are that tan δ is near the allowable limit and the crystallization temperature Tc is slightly high (890° C.). On the contrary, in OG307 where the concentration of SiO₂ is lower than 40 wt. % and the concentration of CaO is higher than 45 wt. %, there is no problem on tan δ and the crystallization temperature. However, there are the problems in the manufacturing process. For example, a glass layer is formed on the surface during the co-firing operation and a fusion to a firing setter (shelf board-shaped or floor board-shaped ceramic refractory used for firing) is caused. In OG310, OG311, OG315 and OG320 where the concentration of B₂O₃ is lower than 9.0 wt. %, the crystallinity is high, the contraction caused by crystallization is large and the remaining glass phase is small. Thus, the deviation from the contraction behavior of the silver electrode becomes large and a large warpage is generated on the LTCC substrate. In OG322 where the concentration of CaO is lower than 40 wt. %, the relative permittivity is 4.5 which is a small value. The relative permittivity becomes low even though the β-wollastonite is not deposited because the sinterability is not good (a lot of pores exist). This point is also confirmed by an electron microscope photograph. In OG315 and OG320 where the concentration of SiO₂ is higher than 46.0 wt. % and the concentration of B₂O₃ is lower than 9.0 wt. %, the remaining glass phase is small and the contraction caused by crystallization is large. Thus, a warpage is generated. In addition, when the concentration of SiO₂ is higher than 46.0 wt. % and the concentration of CaO is lower than 40.0 wt. %, the obtained glass is devitrified. Thus, this composition is not listed in Table 1.

TABLE 1 glass composition code (a) (b) (c) (d) glass code number OG301 OG325 OG323 OG326 OG302 OG303 OG305 composition SiO₂ 45.65 43.47 40.12 43.74 39.81 34.17 42.70 [wt %] CaO 42.60 43.47 40.12 43.74 37.14 31.89 39.84 B₂O₃ 11.75 11.61 18.42 11.67 23.05 33.94 17.46 Al₂O₃ 0.00 0.61 0.57 0.00 0.00 0.00 0.00 ZrO₂ 0.00 0.64 0.59 0.65 0.00 0.00 0.00 MgO 0.00 0.20 0.18 0.20 0.00 0.00 0.00 DTA glass transition 680 675 660 670 670 660 670 [° C.] temperature Tg softening point Ts 780 770 755 770 — 865 785 crystallization 850 830 855 815 890 925 890 temperature Tc relative permittivity k 6.2 6.4 6.7 6.5 3.9 6.7 6.4 dielectric tangent (tan δ) 0.0003 0.0011 0.0018 0.0006 0.0330 0.0260 0.0019 glass composition code glass code number OG307 OG310 OG311 OG315 OG320 OG322 composition SiO₂ 35.33 46.18 45.03 48.66 48.17 42.94 [wt %] CaO 47.12 47.87 42.03 45.40 44.96 37.30 B₂O₃ 17.55 5.95 8.70 5.94 5.88 18.42 Al₂O₃ 0.00 0.00 4.24 0.00 0.98 0.57 ZrO₂ 0.00 0.00 0.00 0.00 0.00 0.59 MgO 0.00 0.00 0.00 0.00 0.00 0.18 DTA glass transition 660 705 685 690 700 660 [° C.] temperature Tg softening point Ts 760 820 810 795 810 865 crystallization 800 840 875 820 850 925 temperature Tc relative permittivity k 6.6 6.5 7.6 6.4 6.4 4.5 dielectric tangent (tan δ) 0.0008 0.0003 0.0031 0.0004 0.0008 0.0014

In the present invention, the β-wollastonite crystal is deposited as a main crystal phase from CaO-B₂O₃-SiO₂ based glass having the composition of (a), (b), (c) and (d) selected in the previous paragraph by using β-wollastonite powder which is the filler is used as a crystallization agent. Note that CaB₂O₄ crystal may be partly deposited as an auxiliary crystal phase in some cases. β-wollastonite is more preferable than α-wollastonite because the relative permittivity k is lower. It is enough to add 1.5 to 3.0 wt. % of the β-wollastonite powder for making the β-wollastonite powder function as a crystal nucleus. If the amount of the β-wollastonite powder is smaller than the above described range, the crystallization effect is reduced. If the amount of the β-wollastonite powder is larger than the above described range, the effect is not changed. In addition, the particle diameter D₅₀ of the β-wollastonite powder is suitably 2.0 to 3.0 μm which is approximately same as the particle diameter D₅₀ of the glass powder A.

In the present invention, the SiO₂ powder which is the filler is used for reducing the relative permittivity k. Furthermore, a part of the SiO₂ powder is taken in a B₂O₃-rich glass phase remained after the crystal is deposited. A part of the SiO₂ powder surrounds the B₂O₃-rich remaining glass phase. Thus, the interaction with the silver electrode can be suppressed and the deterioration of the electrode caused by the migration of the silver can be suppressed. In order to exhibit the above described function of the SiO₂ powder, the inventor found that two or more kinds of nanometer-sized SiO₂ powders having different ranges of particle diameter should be used.

As for the amount of the SiO₂ powder to be added, it is necessary to add 7.5 to 14 wt. % of the SiO₂ powder. If the amount is below 7.5 wt. %, the relative permittivity k exceeds 6.0, the later described yellowing of the silver electrode is remarkable and R₄₂₀/R₈₀₀ is below 85%. On the other hand, if the amount exceeds 14 wt. %, the sinterability in the co-firing process is deteriorated and tan δ is increased (Q value is below 500).

In one embodiment, it is preferable that the SiO₂ powder is formed of 10 to 30 wt. % of the SiO₂ powder having the particle diameter D₅₀ of 10 to 100 nm and 70 to 90 wt. % of the SiO₂ powder having the particle diameter D₅₀ of 400 to 3000 nm. In another embodiment, it is preferable that the SiO₂ powder is formed of 60 wt. % or more of the SiO₂ powder having the particle diameter D₅₀ of 100 to 400 nm and less than 40 wt. % of the SiO₂ powder having the particle diameter D₅₀ of 400 to 3000 nm. In a further embodiment, it is preferable that the SiO₂ powder is formed of 4 to 20 wt. % of the SiO₂ powder having the particle diameter D₅₀ of 10 to 100 nm, 60 to 95 wt. % of the SiO₂ powder having the particle diameter D₅₀ of 100 to 400 nm and 0 to 36 wt. % of the SiO₂ powder having the particle diameter D₅₀ of 400 to 3000 nm. When the composition is not within the above described range, it is difficult to suppress the interaction with the silver electrode and suppress the deterioration of the electrode caused by the migration of the silver while keeping low k value and high Q value.

The interaction between the silver electrode and the remaining glass phase and the migration of the silver to the remaining glass phase can be evaluated by measuring the reflectance of the LTCC substrate after the co-firing process. This is because the silver atoms transferred to the remaining glass phase are colloidal particles and a selective absorption peculiar to the silver colloid occurs near the wavelength of 420 nm and the colloidal particles are observed as the yellowing. Accordingly, the moving amount of the silver atoms to the remaining glass phase can be relatively evaluated from the degree of the yellowing by measuring a ratio R₄₂₀/R₈₀₀ between a reflectance R₈₀₀ in the wavelength of 800 nm not affected by the silver colloidal particles and a reflectance R₄₂₀ where the reflectance is lowered by the absorption of the silver colloidal particles.

By adopting the composition of the present invention, when a silver paste, which functions as an electrode, is applied on a surface of a green sheet comprised of the low temperature co-fired substrate before the co-firing process and an organic binder and the co-firing process is performed, the low temperature co-fired substrate after the co-firing process can have the following properties: a relative permittivity k is 6.0 or less at 2.5 GHz; a Q value which is a reciprocal (1/tan δ) of dielectric tangent is 500 or more at 2.5 GHz; and a ratio R₄₂₀/R₈₀₀ (in percent) of a reflectance R₄₂₀ in a wavelength of 420 nm with respect to a reflectance R₈₀₀ in the wavelength of 800 nm is 85% or more.

Effects of the Invention

In the low temperature co-fired substrate composition of the present invention, the β-wollastonite powder accelerates the deposition of the β-wollastonite crystal in the CaO-B₂O₃-SiO₂ glass, the SiO₂ powder is taken in the B₂O₃-rich glass phase remained after the deposition and the SiO₂ powder surrounds around the remaining glass phase. Thus, the interaction between the B₂O₃-rich remaining glass phase and the silver electrode and the migration of the silver to the B₂O₃-rich remaining glass phase can be suppressed. Consequently, the LTCC substrate can be obtained without causing yellowing of the silver electrode (deterioration of silver electrode) while maintaining low k value and high Q value.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a temperature profile when firing a low temperature co-fired substrate composition of the present invention.

FIG. 2 is a schematic diagram showing a test body and a measurement position for measuring R₄₂₀/R₈₀₀.

FIG. 3 shows a reflectance profile for calculating R₄₂₀/R₈₀₀ of Example 12 of the present invention and Comparative example 19.

FIG. 4 is an example (Example 12) of X-ray diffraction profile of the low temperature co-fired substrate composition of the present invention after the firing process.

FIGS. 5A to 5C are electron microscope photographs showing a surface state (effect of adding the β-wollastonite powder and effect of adding two kinds or more nanometer-sized SiO₂ powder) after the co-firing process.

MODES FOR CARRYING OUT THE INVENTION Screening of Glass Composition

Before exploiting the present invention, screening of glass composition was performed for determining the glass composition suitable for being used for the present invention. First, a green sheet was manufactured from the glass powder by the method shown below (without adding the SiO₂ powder and the β-wollastonite) and the green sheet was fired by the temperature profile shown in FIG. 1 . Thus, the substrate after the firing process was obtained. The relative permittivity k and tan δ (Q value) of the substrate after the firing process were measured. The result is shown in Table 1. The glass composition codes (a), (b), (c) and (d) were selected as the glass powder A for the reasons described in the paragraph [0017].

EXAMPLES Preparation of Glass Powder A

As a raw material, a raw material batch formed by mixing SiO₂, CaCO₃, B₂O₃ or H₃BO₃, Al(OH)₃ or Al₂O₃, ZrO₂ and MgCO₃ to have an oxide ratio (wt. %) shown in (a), (b), (c) or (d) of Table 1 was filled in a platinum crucible, the mixture was melted in an electric furnace in conditions of 1450° C. to 1500° C. and 60 to 180 minutes, and the molten material was entered into water to cool and dry the molten material. Thus, the glass having the compositions shown in (a), (b), (c) or (d) were obtained. The obtained glass was classified using a sieve having a mesh size of 4 mm. The glass passed through the sieve was crushed and classified using an ultrafine grinder incorporating a medium stirring type classifier where the crushing condition was adjusted so that the particle diameter D₅₀ was 2.0 to 3.0 μm. Thus, the glass powder A having the particle diameter D₅₀ shown in FIG. 3 was obtained. The particle diameter D₅₀ was measured by a laser diffraction method using a particle-size-distribution measurement device LA-950V2 manufactured by HORIBA, Ltd.. The measurement result is shown in Table 3.

Preparation of SiO₂ Powder B

Regarding the SiO₂ powder B, AEROSIL (registered trademark) R805 manufactured by NIPPON AEROSIL CO., LTD. was used as the powder of 10 to 100 nm, SO-C1 manufactured by Admatechs Company Limited was used as the powder of 100 to 400 nm, and SO-C2 manufactured by Admatechs Company Limited or Fuselex/X manufactured by TATSUMORI LTD. was used as the powder of 400 to 3000 nm. The compounding ratio of each particle diameter of SiO₂ powder is shown in Table 3.

Preparation of β-wollastonite Powder C

Commercially available crystal powder of β-wollastonite was crushed using a dry ball mill while adjusting the crushing time so that the particle diameter D₅₀ was 2.0 to 3.0 μm and classified using a sieve having a mesh size of 100 μm. Thus, the β-wollastonite powder having the particle diameter D₅₀ of 2.4 μm was obtained.

Manufacturing of Green Sheet for Low Temperature Co-fired Substrate

Each of composition materials A, B and C was weighed so that the ratio (wt. %) shown in Table 3 was satisfied and they are mixed together with an organic binder having the composition (wt. %) shown in Table 2 in a resin-made pot having alumina balls for 16 to 24 hours. Thus, a green sheet precursor slurry was obtained. The amount of the organic binder contained in the slurry was 46 to 49 wt. %. The obtained slurry was deaerated in a vacuum by a rotary pump, a film having a gap of approximately 0.35 mm was formed by a comma direct method and the film was passed through drying zones set to 60° C., 95° C. and 100° C. for three minutes each. Thus, a green sheet having a thickness of approximately 0.12 mm was obtained.

TABLE 2 composition concentration (wt. %) cyclohexanone 48.61 ethanol 32.41 plasticizer 6.4 dispersant 0.85 polyvinyl butyral 11.73

Measurement of Relative Permittivity

Forty layers of the obtained green sheets were laminated by applying a hydrostatic pressure using a hot water laminator/WL28-45-200 manufactured by NIKKISO CO., LTD. in condition that the water temperature of 70° C., the pressure of 20.7 MPa and 10 minutes, and then the green sheets were fired in accordance with the temperature program shown in FIG. 1 . Thus, a low temperature fired substrate was obtained. Here, the temperature rising speed was 1° C./min until 450° C. and the temperature was kept at 450° C. for 2 hours to perform a debinder treatment. After that the temperature was raised to 850° C. in 2 hours and the temperature was kept at 850° C. for 15 minutes to perform a crystallization treatment. Note that the firing process was performed by using a muffle furnace manufactured by MOTOYAMA CO., LTD while introducing approximately 100 L/min of air. The obtained low temperature fired substrate was processed to have a shape of approximately 3×4×30 mm. Then, the relative permittivity k and dielectric tangent (tan δ) were measured by a perturbation type cavity resonator method. The Q value was calculated in condition that Q=1/tan δ. Note that dielectric characteristics were measured by using a relative permittivity/dielectric tangent measurement system TMR-2A manufactured by KEYCOM Corporation. The measurement result is shown in Table 3.

Measurement of R₄₂₀/R₈₀₀

The obtained green sheet was cut into 22×22 mm and a silver paste having a dimeter of 8 mm was applied on the center of the green sheet by screen printing. Five layers of green sheets were laminated so that the applied silver pastes were overlapped with each other and the green sheets were co-fired. Thus, a measuring specimen was obtained (FIG. 2 ). The reflectance was measured on the silver electrode of the obtained specimen by using a spectrophotometer/V-750 manufactured by JASCO Corporation within the range of 200 to 800 nm. Then, R₄₂₀/R₈₀₀ was calculated from the values of the reflectance of 420 nm and 800 nm (FIG. 3 ). The calculation result is shown in Table 3. As understood from FIG. 3 , the deterioration of the reflectance was not observed in 420 nm and R₄₂₀/R₈₀₀ was 88.6% in Example 12, while the deterioration of the reflectance due to the silver colloid was observed in 420 nm and R₄₂₀/R₈₀₀ was deteriorated to 75.2% in Comparative example 19. As the silver paste to be applied, the silver paste manufactured by NORITAKE CO., LIMITED was used.

Identification of Deposited Crystal Phase

The low temperature fired substrate after the firing process was crushed by an alumina magnetic mortar and the measurement was performed within the range of 15 to 35 degrees by using an X-ray diffraction device/X'Pert PRO manufactured by Malvern Panalytical Ltd.. Thus, the deposited crystal phase was identified from the peak positions. An example (Example 12) of the identification result is shown in FIG. 4 . From the identification result, β-wollastonite was detected as the main crystal phase and CaB₂O₄ phase was detected as the auxiliary crystal phase.

The summary of the measurement result of the above described examples is shown in Table 3. When at least two of three kinds of nanometer-sized SiO₂ powder were added at the ratio described in the claim, the interaction between the B₂O₃-rich remaining glass phase and the silver electrode and the migration of the silver to the B₂O₃-rich remaining glass phase could be suppressed (R₄₂₀/R₈₀₀ could be 85.0% or more) while keeping low k value and high Q value.

TABLE 3 Examples 1 2 3 4 5 6 7 glass powder A wt. % 87.30 87.22 87.22 85.48 84.60 87.22 87.22 SiO₂ powder B 9.70 10.78 10.78 12.56 13.46 10.78 10.78 β-wollastonite 3.00 2.00 2.00 1.96 1.94 2.00 2.00 powder C other additives 0.00 0.00 0.00 0.00 0.00 0.00 0.00 particle diameter D₅₀ μm 2.0 2.2 2.2 2.2 2.2 2.2 2.2 of glass powder A glass composition code (a) (b) (b) (b) (b) (b) (b) of glass powder A SiO₂ powder B  10-100 25.00 18.18 27.27 15.92 22.29 0.00 0.00 (particle diameter 1) nm SiO₂ powder B 100-400 0.00 0.00 0.00 0.00 0.00 63.64 81.82 (particle diameter 2) nm SiO₂ powder B 400-3000 75.00 81.82 72.73 84.08 77.71 36.36 18.18 (particle diameter 3) nm particle diameter D₅₀ μm 2.4 2.4 2.4 2.4 2.4 2.4 2.4 of β-wollastonite powder C main crystal phase α or β β β β β β β β (wollastonite) relative at 2.5 5.72 5.93 5.82 5.81 5.59 5.88 5.90 permittivity k GHz tan δ 0.0008 0.0009 0.0011 0.0012 0.0015 0.0009 0.0012 Q value (1/tan) 1250 1111 909 833 667 1111 833 R₄₂₀/R₈₀₀ (%) 88.3 86.4 85.6 87.4 87.6 85.6 89.3 Example 8 9 10 11 12 13 14 15 glass powder A wt. % 87.22 87.22 87.22 86.55 90.16 90.16 90.16 88.20 SiO₂ powder B 10.78 10.78 10.78 8.56 7.84 7.84 7.84 9.80 β-wollastonite 2.00 2.00 2.00 1.98 2.00 2.00 2.00 2.00 powder C Other additive (ZrO₂) 0.00 0.00 0.00 2.91 0.00 0.00 0.00 0.00 particle diameter D₅₀ μm 2.2 2.2 2.2 2.3 2.3 2.3 2.3 2.2 of glass powder A glass composition symbol (b) (b) (b) (c) (c) (c) (c) (d) of glass powder A SiO₂ powder B  10-100 4.55 9.09 13.64 11.68 6.25 12.50 18.75 30.00 (particle diameter 1) nm SiO₂ powder B 100-400 63.64 63.64 63.64 0.00 93.75 87.50 81.25 0.00 (particle diameter 2) nm SiO₂ powder B 400~3000 31.82 27.27 22.73 88.32 0.00 0.00 0.00 70.00 (particle diameter 3) nm particle diameter D₅₀ μm 2.4 2.4 2.4 2.4 2.4 2.4 2.4 2.4 of β-wollastonite powder C main crystal phase α or β β β β β β β β β (wollastonite) relative at 2.5 5.87 5.85 5.73 5.88 5.89 5.72 5.54 5.59 permittivity k GHz tan δ 0.0010 0.0013 0.0017 0.0009 0.0010 0.0011 0.0015 0.0009 Q value (1/tan δ) 1000 769 588 1111 1000 909 667 1111 R₄₂₀/R₈₀₀ (%) 85.2 89.6 89.1 87.1 88.6 88.2 90.6 86.9

Comparative Examples

Comparative examples are shown in Table 4. In Comparative examples 1, 6 and 18, the densities of the glass powder A, the SiO₂ powder B and the β-wollastonite powder C are out of the range of the claim. In other Comparative examples, the range of the particle diameter of the SiO₂ powder B and the content amount of the SiO₂ powder of that range of the particle diameter are out of the range of the claim. In Comparative example 9 and Comparative example 18, the Q value does not reach 500 which is the target value. In other Comparative examples, R₄₂₀/R₈₀₀ is below 85% and remarkable yellowing occurs.

From the result of the Examples and Comparative examples, the SiO₂ powder having a small particle diameter has a large specific surface area and attracts large amount of the B₂O₃-rich remaining glass phase. As a result, the migration of the silver to the B₂O₃-rich glass phase is suppressed. When the SiO₂ powder having a small particle diameter is too much, the structure after the firing process is sparse and tan δ is large although R₄₂₀/R₈₀₀ is large (Comparative example 9). On the contrary, when the SiO₂ powder having a large particle diameter is used, the structure after the firing process is relatively large and tan δ can be kept low. However, when the SiO₂ powder having a large particle diameter is increased, the effect of suppressing the migration of the silver to the B₂O₃-rich glass phase is poor and R₄₂₀/R₈₀₀ is small (Comparative examples 7 and 14). As a result, only when at least two of three kinds of nanometer-sized SiO₂ powder were added at the ratio described in the claim, the interaction between the B₂O₃-rich remaining glass phase and the silver electrode and the migration of the silver to the B₂O₃-rich remaining glass phase could be suppressed while keeping the k value of 6.0 or less and the tan δ of 0.002 or less (Q value of 500 or more).

TABLE 4 Comparative example 1 2 3 4 5 glass powder A wt. % 100.00 90.16 88.20 87.22 86.24 SiO₂ powder B 0.00 7.84 9.80 10.78 11.76 β-wollastonite 0.00 2.00 2.00 2.00 2.00 powder C Other additive 0.00 0.00 0.00 0.00 0.00 particle diameter D₅₀ μm 2.2 2.2 2.2 2.2 2.2 of glass powder A glass composition symbol (b) (b) (b) (b) (b) of glass powder A SiO₂ powder  10-100 0.00 0.00 0.00 0.00 0.00 B (particle diameter 1) nm SiO₂ powder 100-400 0.00 0.00 0.00 0.00 0.00 B (particle diameter 2) nm SiO₂ powder 400-3000 0.00 100.00 100.00 100.00 100.00 B (particle diameter 3) nm particle diameter D₅₀ μm — 2.4 2.4 2.4 2.4 of β-wollastonite powder C main crystal phase α or β α β β β β (wollastonite) relative at 2.5 6.40 6.18 5.96 5.93 5.84 permittivity k GHz tan δ 0.0011 0.0008 0.0009 0.0008 0.0008 Q value (1/tan δ) 909 1250 1111 1250 1250 R₄₂₀/R₈₀₀ (%) 76.8 77.9 78.9 79.3 82.6 Comparative example 6 7 8 9 10 glass powder A wt. % 82.32 87.22 87.22 87.22 86.35 SiO₂ powder B 15.68 10.78 10.78 10.78 11.67 β-wollastonite 2.00 2.00 2.00 2.00 1.98 powder C Other additive 0.00 0.00 0.00 0.00 0.00 particle diameter D₅₀ μm 2.2 2.2 2.2 2.2 2.2 of glass powder A glass composition symbol (b) (b) (b) (b) (b) of glass powder A SiO₂ powder  10-100 0.00 4.55 9.09 63.64 8.57 B (particle diameter 1) nm SiO₂ powder 100-400 0.00 0.00 0.00 0.00 0.00 B (particle diameter 2) nm SiO₂ powder 400-3000 100.00 95.45 90.91 36.36 91.43 B (particle diameter 3) nm particle diameter D₅₀ μm 2.4 2.4 2.4 2.4 2.4 of β-wollastonite powder C main crystal phase α or β β β β β β (wollastonite) relative at 2.5 5.60 5.92 5.90 5.24 5.95 permittivity k GHz tan δ 0.0008 0.0008 0.0008 0.0023 0.0009 Q value (1/tan δ) 1250 1250 1250 435 1111 R₄₂₀/R₈₀₀ (%) 84.5 80.4 84.1 87.4 81.5 Comparative example 11 12 13 14 15 16 glass powder A wt. % 87.22 87.22 87.22 86.35 85.48 87.42 SiO₂ powder B 10.78 10.78 10.78 11.67 12.56 7.64 β-wollastonite 2.00 2.00 2.00 1.98 1.96 2.00 powder C Other additive (ZrO₂) 0.00 0.00 0.00 0.00 0.00 2.94 particle diameter D₅₀ μm 2.2 2.2 2.2 2.2 2.2 2.2 of glass powder A glass composition symbol (b) (b) (b) (b) (b) (c) of glass powder A SiO₂ powder B 10~100 0.00 0.00 0.00 0.00 0.00 0.00 (particle diameter 1) nm SiO₂ powder B 100-400 18.18 36.36 54.55 8.57 15.92 0.00 (particle diameter 2) nm SiO₂ powder B 400-3000 81.82 63.64 45.45 91.43 84.08 100.00 (particle diameter 3) nm particle diameter D₅₀ μm 2.4 2.4 2.4 2.4 2.4 2.4 of β-wollastonite powder C main crystal phase α or β β β β β β β (wollastonite) relative at 2.5 5.95 5.99 5.88 5.94 5.84 5.93 permittivity k GHz tan δ 0.0008 0.0008 0.0009 0.0008 0.0008 0.0008 Q value 1250 1250 1111 1250 1250 1250 (1/tan δ) R₄₂₀/R₈₀₀ (%) 80.8 82.3 82.7 78.4 82.7 79.5 Comparative example 17 18 19 20 21 glass powder A wt. % 86.98 81.30 90.16 90.16 88.20 SiO₂ powder B 8.10 14.11 7.84 7.84 9.80 β-wollastonite 1.99 1.86 2.00 2.00 2.00 powder C Other additive (ZrO₂) 2.93 2.73 0.00 0.00 0.00 particle diameter D₅₀ μm 2.3 2.3 2.2 2.3 2.2 of glass powder A glass composition symbol (c) (c) (c) (c) (d) of glass powder A SiO₂ powder B 10~100 6.17 49.61 0.00 0.00 0.00 (particle diameter 1) nm SiO₂ powder B 100-400 0.00 0.00 0.00 50.00 0.00 (particle diameter 2) nm SiO₂ powder B 400-3000 93.83 50.39 100.00 50.00 100.00 (particle diameter 3) nm particle diameter D₅₀ μm 2.4 2.4 2.4 2.4 2.4 of β-wollastonite powder C main crystal phase α or β β β β β β (wollastonite) relative at 2.5 5.91 4.19 5.96 5.79 5.94 permittivity k GHz tan δ 0.0009 0.0028 0.0007 0.0008 0.0005 Q value 1111 357 1429 1250 2000 (1/tan δ) R₄₂₀/R₈₀₀ (%) 81.6 91.3 75.2 80.2 82.6

Finally, the effect of the present invention was confirmed based on the result of observing a surface state of the LTCC substrate after the co-firing process by an electron microscope photograph (FIGS. 5A to 5C). When Comparative example 1 containing only the glass powder without containing the SiO₂ powder and the β-wollastonite powder was co-fired, it was confirmed that the floating of the B₂O₃-rich glass phase densely existed on the surface in a grained state. In such a case, the migration of the silver to the B₂O₃-rich glass phase was remarkable. Thus, R₄₂₀/R₈₀₀ was deteriorated and disconnection or short circuiting of the electrode may occur. In the example where the β-wollastonite powder is added to Comparative example 1, although the floating of the B₂O₃-rich glass was reduced but did not completely disappear. In Example 14 where two or more kinds of nanometer-sized SiO₂ powders having different ranges of particle diameter are added in addition to the β-wollastonite, it was confirmed that the floating of the B₂O₃-rich glass phase completely disappeared and the surface was fine and smooth. 

What is claimed is:
 1. A low temperature co-fired substrate composition before a co-firing process, comprising: (A) 83 to 91 wt. % of glass powder containing CaO-B₂O₃-SiO₂ as a basic composition; (B) 7.5 to 14 wt. % of SiO₂ powder; and (C) 1.5 to 3 wt. % of β-wollastonite (CaSiO₃) powder, wherein a particle diameter D₅₀ of the glass powder (A) is 2.0 to 3.0 μm, the glass powder (A) contains 40.0 to 45.0 wt. % of CaO, 9.0 to 20.0 wt. % of B₂O₃ and 40.0 to 46.0 wt. % of SiO₂ as a composition, the SiO₂ powder (B) is a filler selected from the group consisting of (1), (2) and (3): (1) 10 to 30 wt. % of the SiO₂ powder having the particle diameter D₅₀ of 10 to 100 nm and 70 to 90 wt. % of the SiO₂ powder having the particle diameter D₅₀ of 400 to 3000 nm; (2) 60 wt. % or more of the SiO₂ powder having the particle diameter D₅₀ of 100 to 400 nm and less than 40 wt. % of the SiO₂ powder having the particle diameter D₅₀ of 400 to 3000 nm; and (3) 4 to 20 wt. % of the SiO₂ powder having the particle diameter D₅₀ of 10 to 100 nm, 60 to 95 wt. % of the SiO₂ powder having the particle diameter D₅₀ of 100 to 400 nm and 0 to 36 wt. % of the SiO₂ powder having the particle diameter D₅₀ of 400 to 3000 nm, the β-wollastonite (CaSiO₃) powder (C) is another filler having the particle diameter D₅₀ of 2.0 to 3.0 μm, when a silver paste, which functions as an electrode, is applied on a surface of a green sheet comprised of the low temperature co-fired substrate before the co-firing process and an organic binder and the co-firing process is performed, the low temperature co-fired substrate after the co-firing process has the following properties: a relative permittivity k is 6.0 or less at 2.5 GHz; a Q value which is a reciprocal of dielectric tangent is 500 or more at 2.5 GHz; and a ratio R₄₂₀/R₈₀₀ of a reflectance R₄₂₀ in a wavelength of 420 nm with respect to a reflectance R₈₀₀ in the wavelength of 800 nm is 85% or more. 